If the wheels of a motor vehicle are locked upon application of the brake of the motor vehicle, rotation of the tire of the vehicle is braked, followed by tire slipping on the road surface.
Such a condition means less vehicle controllability. Antilock braking systems (ABS) are control systems capable of preventing the tires from locking during a braking process.
Generally the conventional antilock braking system for motor vehicles employs a system such that the braking action is automatically controlled in accordance with acceleration of the vehicle wheel. Acceleration speed of the vehicle wheel is determined by taking the derivative of wheel speed which is measured with the use of a wheel speed or wheel velocity sensor. This wheel velocity sensor has a structure measuring rotational velocity of the vehicle wheel by detecting the rotation speed of a geared hub attached on the axle of the wheel with the use of a magnetic pick-up. To minimize error due to dust that adheres on the surface of the geared hub there is a limitation for the fineness in tooth pitch. Consequently, measurement errors due to such wheel speed sensors can be safely ignored for high velocity rotation of the wheel, but for low velocity rotation of the wheel, measurement accuracy will deteriorate because a signal from the magnetic pick-up comes to have a longer input interval.
Control characteristics of motor vehicles with antilock braking systems operated by the measured values of acceleration of the vehicle wheel have a high possibility that controllability becomes unstable when the motor vehicle is traveling at a lower speed, since the measured values show deteriorated accuracy when the wheel is rotating at a lower velocity.